Gluconeogenesis Flashcards
What tissues rely on glucose solely as their main source of energy?
brain nervous system RBCs testes embryonic tissues
What is gluconeogenesis?
generation of glucose from non-carbohydrate substrates e.g pyruvate, lactate, glycerol and glucogenic amino acids
Where does gluconeogenesis normally occur?
the liver
Gluconeogenesis is not the reverse of glycolysis as glycolysis has 3 irreversible steps. What happens instead?
there are 4 bypass reactions A,B,C and D
What step of glycolysis are reactions A and B concerned with bypassing?
Step 10 - conversion of pyruvate or lactate to PEP
What happens in reaction A?
conversion of pyruvate or lactate to oxaloacetate
Lactate is covered to oxaloacetate in 2 steps. What are they?
lactate converted to pyruvate first by lactate dehydrogenase
pyruvate converted to oxaloacetate by pyruvate carboxlase
How is reaction A different if pyruvate is used as the original substrate?
oxaloacetate is converted to malate to leave the mitochondrion and then is reconverted to oxaloacetate once outside the mitochondrion
What happens in Reaction B of gluconeogenesis?
oxaloacetate is converted to PEP
What difference is there in the progression of reaction B of gluconeogenesis when pyruvate is used as a substrate rather than lactate?
lactate - oxaloactetate is converted to PEP INSIDE the mitochondrion
pyruvate - oxaloacetate is converted to PEP outside the mitochondrion
What step of glycolysis is Reaction C concerned with bypassing?
conversion of F-1,6BP to F-6-P
What enzyme catalyses Reaction C?
fructose 1,6-bisphosphatase
What reaction of glycolysis is Reaction D concerned with bypassing?
conversion of G-6-P to glucose
What enzyme catalyses Reaction D?
G-6-Phosphatase
How is free glucose formed?
Consider:
1. What is the usual end point for gluconeogenesis?
2. Where will free glucose be formed?
gluconeogenesis usually stops at G-6-P so that glucose can be ‘trapped’ in the cells
so free glucose formation will take place in the lumen of the ER
Where can fructose enter glycolysis?
at F-6-P
How is fructose metabolised?
by the fructose-1-phosphate pathway
catalysed by 2 enzymes: fructose-1-phosphate aldolase
triose kinase
1 or 2 ATP used per fructose molecule converted
Where can galactose enter glycolysis?
at G-6-P
How is galactose initially metabolised?
converted to G-1-P through sugar nucleotide derivative, UDP galactose
What happens in the pentose phosphate pathway?
produces NADPH produces pentoses (5C sugars) that are precursors of ATP, RNA and DNA
The pentose-phosphate pathway has 2 phases. What are they?
- oxidative, irreversible bit that generates NADPH and converts G-6-P to a pentose phosphate
- an non-oxidative, reversible bit that in converts G-6-P and pentose phosphate to form lots of different 3,4,5,6, and 7 carbon sugars
What enzyme is involved in the irreversible step of P-P-P?
G-6-P dehydrogenase
What is the function of NADPH?
to link catabolic and anabolic pathways
catabolic= pentose phosphate pathway picks up H
anabolic= fatty acid synthesis uses H
What is the difference between NADP+ and NAD+?
they are both electron carriers
NAD+ is used in metabolism
NADP+ is used in anabolism
